(1) Field of the Invention
The present invention relates to a method of forming contact holes used in semiconductor devices.
(2) Description of the Related Art
By a traditional method shown in FIG. 3, an N.sup.+ diffused layer 22 in which is diffused an N-type impurity element such as phosphorus, arsenic or antimony is formed in a silicon substrate 21, an insulating film 23 such as a silicon oxide film is formed thereon, and a contact hole 24 is formed therein in order to bring the N.sup.+ diffused layer 22 and an upper wiring layer 26 into contact with each other. When the size of the contact hole 24 becomes as small as 1 .mu.m or less, it becomes very difficult to make a contact with the upper wiring layer. That is, the upper wiring layer 26 that is formed does not sufficiently enter the contact hole 24 and sufficient contact is not maintained between the N.sup.+ diffused layer 22 and the upper wiring layer 26. Furthermore, the contact hole 24 contains a region 27 in which the upper wiring layer 26 does not enter, i.e., contains a void. In order to improve these problems, there has been proposed a method that is shown in FIGS. 4(a) to 4(e).
Referring to FIG. 4(a), first, an N.sup.+ diffused layer 32 is formed in a silicon substrate 31, an insulating film 33 is formed thereon, and a contact hole 34 is formed therein. Next, as shown in FIG. 4(b), a non-doped polycrystalline silicon film 35 is deposited to completely fill the contact hole 34. The polycrystalline silicon film is formed by a chemical vapor deposition method (CVD method) and accordingly has a good step-covering property, and is capable of sufficiently filling the contact hole even when its size is as small as 1 .mu.m or less. Next, as shown in FIG. 4(c), the polycrystalline silicon film 35 is so etched back that the contact hole 34 only is filled with the polycrystalline silicon film 35. Then, as shown in FIG. 4(d), ions of N-type impurity element such as phosphorus (P), arsenic (As) or antimony (Sb) are implanted in order to introduce the N-type impurity element into the polycrystalline silicon film 35 that exists in the contact hole. Thereafter, heat treatment is carried out and, then, the upper wiring layer 36 is formed as shown in FIG. 4(e) maintaining the contact with the N.sup.+ diffused layer 32.
The problem to be solved by the method of FIG. 4 resides in the implantation of ions shown in FIG. 4(d). That is, in order to sufficiently lower the contact resistance with respect to the N.sup.+ diffused layer 32, the polycrystalline silicon film 35 must have a very low resistance. It therefore becomes necessary to implant the ions in a dosage of 1.times.10.sup.16 /cm.sup.2 or greater, requiring a long period of time for ion implantation and an increased cost. Moreover, the contact hole 34 is deeper than 0.5 .mu.m, and a large acceleration energy or a great load is required for implanting ions deep into the bottom of the polycrystalline silicon film 35. Moreover, though not shown in FIG. 4, a step of masking such as of photo-resist is necessary if it is not desired to implant ions into the regions other than the contact hole 34, resulting in a great increase in the number of steps and in the cost. Furthermore, the implantation of ions damages the semiconductor device to seriously affect its characteristics and reliability.
Moreover, when the insulating film 33 is an insulating film containing P-type impurity elements, for example, a silicon oxide film (BSB film) containing boron or a silicon oxide film (BPSG film) containing boron and phosphorous, the P-type impurity element may diffuse from the insulating film 33 into the polycrystalline silicon film 35 filled in the contact hole, resulting in an increase in the contact resistance, and making it necessary to further increase the dosage.
By a traditional method shown in FIG. 5, a P.sup.+ diffused layer 42 in which is diffused a P-type impurity element such as boron is formed in a silicon substrate 41, an insulating film 43 such as a silicon oxide film is formed thereon, and a contact hole 44 is formed therein in order to bring the P.sup.+ diffused layer 42 and an upper wiring layer 46 into contact with each other. When the size of the contact hole 44 becomes as small as 1 .mu.m or less, it becomes very difficult to make a contact with the upper wiring layer. That is, the upper wiring layer 46 that is formed does not sufficiently enter the contact hole 44 and sufficient contact is not maintained between the P.sup.+ diffused layer 42 and the upper wiring layer 46. Furthermore, the contact hole 44 contains a region 47 which the upper wiring layer 46 does not enter, i.e., contains a void. In order to improve these problems, there has been proposed a method that is shown in FIGS. 6(a) to 6(e).
Referring to FIG. 6(a), first, a P.sup.+ diffused layer 52 is formed in a silicon substrate 51, an insulating film 53 is formed thereon, and a contact hole 54 is formed therein. Next, as shown in FIG. 6(b), a non-doped polycrystalline silicon film 55 is deposited to completely fill the contact hole 54. The polycrystalline silicon film is formed by the chemical vapor deposition method (CVD method) and accordingly has a good step-covering property, and is capable of sufficiently filling the contact hole even when its size is as small as 1 .mu.m or less. Next, as shown in FIG. 6(c), the polycrystalline silicon film 55 is so etched back that the contact hole 54 only is filled with the polycrystalline silicon film 55. Then, as shown in FIG. 6(d), ions of P-type impurity element such as boron (B) are implanted in order to introduce P-type impurity element into the polycrystalline silicon film 55 that exists in the contact hole. Thereafter, the heat treatment is carried out and, then, the upper wiring layer 56 is formed as shown in FIG. 6(e) maintaining the contact with the P.sup.+ diffused layer 52.
The problem to be solved by the method of FIG. 6 resides in the implantation of ions shown in FIG. 6(d). That is, in order to sufficiently lower the contact resistance with respect to the P.sup.+ diffused layer 52, the polycrystalline silicon film 55 must have a very low resistance. It therefore becomes necessary to implant the ions in a dosage of 1.times.10.sup.16 /cm.sup.2 or greater, requiring a long period of time for ion implantation and an increased cost. Moreover, the contact hole 54 is deeper than 0.5 .mu.m, and a large acceleration energy or a great load is required for implanting ions deep into the bottom of the polycrystalline silicon film 55. Moreover, though not shown in FIG. 6, a step of masking such as of photo-resist is necessary if it is not desired to implant ions into the regions other than the contact hole 54, resulting in a great increase in the number of steps and in the cost. Furthermore, the implantation of ions damages the semiconductor device to seriously affect its characteristics and reliability.
Moreover, when the insulating film 53 is an insulating film containing N-type impurity elements, for example, a silicon oxide film (PSG film) containing phosphorus, a silicon oxide film (BPSG film) containing boron and phosphorus or a silicon oxide film (ASG film) containing arsenic, the N-type impurity element may diffuse from the insulating film 53 into the polycrystalline silicon film 55 filled in the contact hole, resulting in an increase in the contact resistance, and making it necessary to further increase the dosage of ion injection.